If my widget button is round and not a square, how do i make my mouse detect the widget for round edges and not as a square?
Anyone? 
make my mouse detect the widget for round edges
You don’t, not in pure UMG. 
The only plugin I know of that handles this - Widget Irregular Button. It was far from (pixel) perfect when released but I see great reviews now and it’s being kept up to date.
Blueprint / UMG alternatives:
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approximate - take 6-24 transparent borders and rotate them around a pivot - almost a circle. Janky and inefficient but no one will notice. Works for other shapes, too. Glue some rectangular borders together.
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for a circle, you could check the distance to the cursor on Tick - that’d be a perfect circle.
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the HUD class can create hit boxes, math heavier but should be faster that aproximating with borders:
The latter is somewhat of an archaic / arcane knowledge - not too much documentation exists, and probably few tutorials.